Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly, to an exhaust gas purifying apparatus having a NOx absorbing catalyst and a NOx concentration sensor which are provided in an exhaust passage of the internal combustion engine.
Description of the Related Art
Japanese Patent Laid-open publication No. 2002-180865 (JP'865) discloses an exhaust gas purifying apparatus in which a NOx absorbent is disposed in an exhaust passage of the internal combustion engine and a NOx concentration sensor (of the type that also reacts to ammonia) is disposed downstream of the NOx absorbent. In this apparatus, when performing the rich spike for temporarily enriching the air-fuel ratio for discharging NOx absorbed in the NOx absorbent, an amount of residual reducing agent is obtained based on changes in the ammonia concentration detected by the NOx concentration sensor. The detected value which is obtained, by the NOx concentration sensor, in the vicinity of the start timing of the rich spike, is regarded as a NOx concentration, while the detected value obtained in the vicinity of the end timing of the rich spike is regarded as an ammonia concentration. As shown in JP'865, it is known that the detected output from the NOx concentration sensor having an ion conductive solid electrolyte layer containing zirconia is proportional not only to the NOx concentration but also to a concentration of ammonia generated by performing the rich spike. In this specification, the “NOx concentration sensor” means a NOx concentration sensor having the ion conductive solid electrolyte layer containing zirconia.
According to the apparatus shown in JP'865, the detected output in the vicinity of the end timing of the rich spike is regarded as the ammonia concentration. However, the time period during which the reducing agent supplied by the rich spike reacts with NOx to generate ammonia, may not necessarily be constant. Accordingly, if the determination whether the detected output indicates the ammonia concentration or the NOx concentration is performed based only on the timing relationship with the rich spike end timing, the determination cannot be performed with high accuracy. For example, if it is determined that the detected output during a constant time period TNH3 from the rich spike end timing indicates the ammonia concentration, and the constant time period TNH3 is set to a too short time period, the ammonia concentration may erroneously be regarded as the NOx concentration after the constant time period TNH3 has elapsed, and the rich spike may be performed again immediately after the rich spike ends. On the other hand, if the constant time period TNH3 is set to a too long time period, the problem described below may occur. That is, in the state where the absorbing capacity of the NOx absorbent is reduced, NOx starts to flow out downstream of the NOx absorbent at a comparatively early timing after the rich spike ends. Accordingly, the NOx concentration may erroneously be regarded as the ammonia concentration, which may cause erroneous determination of the start timing of the next rich spike.